34677964 four wheel steering report

8/11/2019 34677964 Four Wheel Steering Report

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FOUR WHEEL STEERING SYSTEM

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A2STRA3T

The four wheel steering control with rear wheel drive is a new concept which can be

implemented in four wheeler vehicles. Four-wheel steering is a system can employed in

some vehicles to improve steering response, increase vehicle stability while moving at

certain speed, or to decrease turning radius at low speed.

In four-wheel steering systems, the rear wheels are control by a gear arrangement.

This allows the vehicle to turn in a significantly smaller radius sometimes critical for large

trucks or tractors and vehicles with trailers.

INTRO"U3TION

Four-wheel steering, 4W, also called rear-wheel steering or all-wheel

steering, provides a means to actively steer the rear wheels during turning maneuvers. It

should not be confused with four-wheel drive in which all four wheels of a vehicle are

powered. It improves handling and helps the vehicle make tighter turns.

!roduction-built cars tend to understeer or, in few instances, oversteer. If a car

could automatically compensate for an understeer"oversteer problem, the driver would en#oy

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nearly neutral steering under varying conditions. 4W is a serious effort on the part of

automotive design engineers to provide near-neutral steering.

The front wheels do most of the steering. %ear wheel turning is generally limited to

half during an opposite direction turn. When both the front and rear wheels steer toward the

same direction, they are said to be in-phase and this produces a kind of sideways movement

of the car at low speeds. When the front and rear wheels are steered in opposite direction,

this is called anti-phase, counter-phase or opposite-phase and it produces a sharper, tighter

turn.

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OVERVIEW

To reduce turning radius

'se of 4-wheel steering system.

&($ front to rear wheel angle ratio.

)onsiderations(

*river performance and comfort.

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ust not interfere with other components.

!roper stability of car during competition.

3ON3E4TS

ow to achieve front to rear ratio

/eometry( 0asier to construct, more reliable, less components.

/earing( 0asier to design, identical geometry and components for subsystems.

4RIN3I4LE

The steering mechanism consists of rack and pinion arrangements which are used to

turn the wheels in the front. 1nd a bevel gear arrangement is made #ust after the steering and

power is transmitted through the transfer shaft to the gear bo2 assembly. Then power is

transmitted to the rear wheels.

3ayout"peration of the system( Two subsystems( %ack and pinion for front and rear,

identical geometry and components. teering column is fitted with + bevel gears meshes and

transmits power to front and rear rack and pinion. 1s steering wheel is turned the entire

rotation is transferred to front rack and pinion and only half of the rotation is transferred to

rear rack and pinion.

WHY FOUR5WHEEL STEERING SYSTEM6

To understand the advantages of four-wheel steering, it is wise to review the

dynamics of typical steering maneuvers with a conventional front -steered vehicle. The tires

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are sub#ect to the forces of grip, momentum, and steering input when making a movement

other than straight-ahead driving. These forces compete with each other during steering

maneuvers. With a front-steered vehicle, the rear end is always trying to catch up to the di-

rectional changes of the front wheels. This causes the vehicle to sway. 1s a normal part of

operating a vehicle, the driver learns to ad#ust to these forces without thinking about them.

When turning, the driver is putting into motion a comple2 series of forces. 0ach of

these must be balanced against the others. The tires are sub#ected to road grip and slip angle.

/rip holds the car5s wheels to the road, and momentum moves the car straight ahead.

teering input causes the front wheels to turn. The car momentarily resists the turning

motion, causing a tire slip angle to form. nce the vehicle begins to respond to the steering

input, cornering forces are generated. The vehicle sways as the rear wheels attempt to keep

up with the cornering forces already generated by the front tires. This is referred to as rear-

end lag, because there is a time delay between steering input and vehicle reaction. When the

front wheels are turned back to a straight -ahead position, the vehicle must again try to

ad#ust by reversing the same forces developed by the turn. 1s the steering is turned, the

vehicle body sways as the rear wheels again try to keep up with the cornering forces

generated by the front wheels.

The idea behind four-wheel steering is that a vehicle re6uires less driver input

for any steering maneuver if all four wheels are steering the vehicle. 1s with two-wheel

steer vehicles, tire grip holds the four wheels on the road. owever, when the driver turns

the wheel slightly, all four wheels react to the steering input, causing slip angles to form at

all four wheels. The entire vehicle moves in one direction rather than the rear half

attempting to catch up to the front. There is also less sway when the wheels are turned back

to a straight-ahead position. The vehicle responds more 6uickly to steering input because

rear wheel lag is eliminated.

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FAIL5SAFE MEASURES

1ll 4W systems have fail-safe measures. For e2ample, with the electro-

hydraulic setup, the system automatically counteracts possible causes of failure( both

electronic and hydraulic, and converts the entire steering system to a conventional two-

wheel steering type. pecifically, if a hydraulic defect should reduce pressure level 8by amovement malfunction or a broken driving belt9, the rear-wheel-steering mechanism is

automatically locked in a neutral position, activating a low-level warning light.

In the event of an electrical failure, it would be detected by a self-diagnostic

circuit integrated in the four wheel-steering control unit. The control unit stimulates a

solenoid valve, which neutrali:es hydraulic pressure, thereby alternating the system to two-

wheel steering. The failure would be indicated by the system5s warning light in the main

instrument display.

n any 4W system, there must be near-perfect compliance between the

position of the steering wheel, the position of the front wheels, and the position of the rear

wheels. It is usually recommended that the car be driven about &; feet 8< meters9 in a dead-

straight line. Then, the position of the front"rear wheels is checked with respect to steering

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wheel position. The base reference point is a strip of masking tape on the steering wheel hub

and the steering column.

When the wheel is positioned dead center, draw a line down the tape. %un the car a

short distance straight ahead to see if the reference line holds. If not, corrections are needed,

such as repositioning the steering wheel. 0ven severe imbalance of a rear wheel on a speed

sensitive 4W system can cause problems and make basic troubleshooting a bit frustrating.

A"VANTAGES OF 7WS

The vehicle5s cornering behavior becomes more stable and controllable at high speeds

as well as on wet or slippery road surfaces.

The vehicle5s response to steering input becomes 6uicker and more precise throughout

the vehicle5s entire speed range.

The vehicle5s straight-line stability at high speeds is improved. =egative effects of

road irregularities and crosswinds on the vehicle5s stability are minimi:ed.

tability in lane changing at high speeds is improved. The vehicle is less likely to go

into a spin even in situations in which the driver must make a sudden and relatively large

change of direction.

>y steering the rear wheels in the direction opposite the front wheels at low speeds,

the vehicle5s turning circle is greatly reduced. Therefore, vehicle maneuvering on narrow

roads and during parking becomes easier.

A44LI3ATIONS OF 7WS

It is used for easy parking of four wheelers, and can be implemented in monster trucks, large

farm vehicles and trucks. 0tc..,

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3ON3LUSION

Thus the four-wheel steering system has got cornering capability, steering

response, straight-line stability, lane changing and low-speed maneuverability. 0ven though

it is advantageous over the conventional two-wheel steering system, 4W is comple2 and

e2pensive. )urrently the cost of a vehicle with four wheel steering is more than that for a

vehicle with the conventional two wheel steering. Four wheel steering is growing in

popularity and it is likely to come in more and more new vehicles. 1s the systems become

more commonplace the cost of four wheel steering will drop.

REFEREN3ES

$. @1utomobile 0ngineeringA, B.%./ovindan

&. http(CCwww.howstuffworks.com

+. http(CCwww.howhurricaneworks.com

4. http(CCwww.thecarconnection.com

7. http(CCwww.wikepedia.com

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34677964 four wheel steering report

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